2. Chapter 26
D.C.
Generators

3. D.C. Generators C-26 Generator Principal Simple Loop Generator
Practical Generator
Yoke
Pole Cores and Pole Shoes
Pole Coils
Armature Core
Armature Windings
Commutator
Brushes and Bearings
Contd….

4. D.C. Generators C-26 Pole-pitch Conductor Coil and Winding Element
Coil-span or Coil-pitch (Ys)
Pitch of a Winding (Y)
Back Pitch (YB)
Front Pitch (YF)
Resultant Pitch (YR)
Commutator Pitch (YG)
Single-layer Winding
Two-layer Winding
Contd….

5. D.C. Generators C-26 Degree of Re – entrancy of an Armature Winding
Multiplex Winding
Lap and Wave Winding
Simplex-lap Winding
Numbering of Coils and Commutator Segments
Simplex Wave Winding
Dummy or Idle Coils
Uses of Lap and Wave Windings
Types of Generators
Brush Contact Drop
Generated E.M.F. or E.M.F. Equation of a Generator
Contd….

6. D.C. Generators C-26 Iron Loss in Armature
Total loss in a D.C. Generator
Stray Losses
Constant or Standing Losses
Power Stages
Condition for Maximum Efficiency

7. Generator Principle
An electrical generator is a machine which converts mechanical energy (or power) into electrical energy (or power).
Two basic essential parts of an electrical generator are (i) a magnetic field and (ii) a conductor or conductors which can so move as to cut the flux.

8. Simple Loop Generator Construction
In Fig is shown a single-turn rectangular copper coil ABCD rotating about its own axis in a magnetic field provided by either permanent magnet is or electromagnets.
The two ends of the coil are joined to two slip-rings ‘a’ and ‘b’ which are insulated from each other and from the central shaft. Two collecting brushes (of carbon or copper) press against the slip-rings.
The rotating coil may be called ‘armature’ and the magnets as ‘field magnets’.
Contd….

9. Simple Loop Generator
Contd….

10. Simple Loop Generator Working
Imagine the coil to be rotating in clock-wise direction (Fig. 26.2). As the coil assumes successive positions in the field, the flux linked with it changes. Hence, an e.m.f. is induced in it which is proportional to the rate of change of flux linkages (e = NdΦdt). When the plane of the coil is at right angles to lines of flux i.e. when it is in position, 1, then flux linked with the coil is maximum but rate of change of flux linkages is minimum.
As the coil continues rotating further, the rate of change of flux linkages (and hence induced e.m.f. in it) increases, till position 3 is reached where θ = 90º. Here, the coil plane is horizontal i.e. parallel to the lines of flux. As seen, the flux linked with the coil is minimum but rate of change of flux linkages is maximum. Hence, maximum e.m.f. is induced in the coil when in this position (Fig. 26.3).
Contd….

11. Simple Loop Generator

12. Practical Generator
The simple loop generator has been considered in detail merely to bring out the basic principle underlying construction and working of an actual generator illustrated in Fig which consists of the following essential parts :
1. Magnetic Frame or Yoke 2. Pole-Cores and Pole-Shoes
3. Pole Coils or Field Coils 4. Armature Core
5. Armature Windings or Conductors 6. Commutator
7. Brushes and Bearings
Contd….

13. Practical Generator

14. Yoke The outer frame or yoke serves double purpose :
It provides mechanical support for the poles and acts as a protecting cover for the whole machine and
It carries the magnetic flux produced by the poles.

15. Pole Cores and Pole Shoes
The field magnets consist of pole cores and pole shoes. The pole shoes serve two purposes
They spread out the flux in the air gap and also, being of larger cross-section, reduce the reluctance of the magnetic path
They support the exciting coils (or field coils).
Contd….

16. Pole Cores and Pole Shoes

17. Pole Coils
The field coils or pole coils, which consist of copper wire or strip, are former-wound for the correct dimension (Fig ). Then, the former is removed and wound coil is put into place over the core as shown in Fig
When current is passed through these coils, they electromagnetise the poles which produce the necessary flux that is cut by revolving armature conductors.
Contd….

18. Pole Coils

19. Armature Core
It houses the armature conductors or coils and causes them to rotate and hence cut the magnetic flux of the field magnets. In addition to this, its most important function is to provide a path of very low reluctance to the flux through the armature from a N- pole to a S-pole.
It is cylindrical or drum-shaped and is built up of usually circular sheet steel discs or laminations approximately 0.5 mm thick (Fig ). It is keyed to the shaft.
Up to armature diameters of about one metre, the circular stampings are cut out in one piece as shown in Fig But above this size, these circles, especially of such thin sections, are difficult to handle because they tend to distort and become wavy when assembled together.
Contd….

20. Armature Core

21. Armature Windings The armature windings are usually former-wound.
These are first wound in the form of flat rectangular coils and are then pulled into their proper shape in a coil puller.
Various conductors of the coils are insulated from each other.
The conductors are placed in the armature slots which are lined with tough insulating material.
This slot insulation is folded over above the armature conductors placed in the slot and is secured in place by special hard wooden or fibre wedges.

22. Commutator
The function of the commutator is to facilitate collection of current from the armature conductors.
A sectional view of commutator is shown in Fig whose general appearance when completed is shown in Fig

23. Brushes and Bearings
The brushes whose function is to collect current from commutator, are usually made of carbon or graphite and are in the shape of a rectangular block. These brushes are housed in brush-holders usually of the box-type variety. As shown in Fig , the brush-holder is mounted on a spindle and the brushes can slide in the rectangular box open at both ends.

24. Armature Windings
Now, we will discuss the winding of an actual armature. But before doing this, the meaning of the following terms used in connection with armature winding should be clearly kept in mind.

25. Pole – Pitch It may be variously defined as :
The periphery of the armature divided by the number of poles of the generator i.e. the distance between two adjacent poles.
It is equal to the number of armature conductors (or armature slots) per pole. If there are 48 conductors and 4 poles, the pole pitch is 48/4 = 12.

26. Conductor
The length of a wire lying in the magnetic field and in which an e.m.f. is induced, is called a conductor (or inductor) as, for example, length AB or CD in Fig

27. Coil and Winding Element
With reference to Fig , the two conductors AB and CD along with their end connections constitute one coil of the armature winding.
The coil may be single-turn coil (Fig ) or multi-turn coil (Fig ). A single-turn coil will have two conductors. But a multi- turn coil may have many conductors per coil side. In Fig , for example, each coil side has 3 conductors.
The group of wires or conductors constituting a coil side of a multi-turn coil is wrapped with a tape as a unit (Fig ) and is placed in the armature slot.
Contd….

28. Coil and Winding Element

29. Coil – Span or Coil – Pitch (YS)
It is the distance, measured in terms of armature slots (or armature conductors) between two sides of a coil. It is, in fact, the periphery of the armature spanned by the two sides of the coil.
If the pole span or coil pitch is equal to the pole pitch (as in the case of coil A in Fig where pole-pitch of 4 has been assumed), then winding is called full-pitched.

30. Pitch of a Winding (Y)
In general, it may be defined as the distance round the armature between two successive conductors which are directly connected together. Or, it is the distance between the beginnings of two consecutive turns.
Y = YB − YF for lap winding
= YB + YF for wave winding

31. Back Pitch (YB)
The distance, measured in terms of the armature conductors, which a coil advances on the back of the armature is called back pitch and is denoted by YB.
As seen from Fig , element 1 is connected on the back of the armature to element 8. Hence, YB = (8 − 1) = 7.

32. Front Pitch (YF)
The number of armature conductors or elements spanned by a coil on the front (or commutator end of an armature) is called the front pitch and is designated by YF. Again in Fig , element 8 is connected to element 3 on the front of the armature, the connections being made at the commutator segment. Hence, YF = 8 − 3 = 5.
Alternatively, the front pitch may be defined as the distance (in terms of armature conductors) between the second conductor of one coil and the first conductor of the next coil which are connected together at the front i.e. commutator end of the armature. Both front and back pitches for lap and wave-winding are shown in Fig and
Contd….

33. Front Pitch (YF)

34. Resultant Pitch (YR)
It is the distance between the beginning of one coil and the beginning of the next coil to which it is connected (Fig and ).
As a matter of precaution, it should be kept in mind that all these pitches, though normally stated in terms of armature conductors, are also sometimes given in terms of armature slots or commutator bars because commutator is, after all, an image of the winding.

35. Commutator Pitch (YG)
It is the distance (measured in commutator bars or segments) between the segments to which the two ends of a coil are connected.
From Fig and it is clear that for lap winding, YC is the difference of YB and YF whereas for wave winding it is the sum of YB and YF. Obviously, commutator pitch is equal to the number of bars between coil leads.
In general, YC equals the ‘plex’ of the lap-wound armature.

36. Single – Layer Winding
It is that winding in which one conductor or one coil side is placed in each armature slot as shown in Fig Such a winding is not much used.

37. Two – Layer Winding
In this type of winding, there are two conductors or coil sides per slot arranged in two layers.
The transfer of the coil from one slot to another is usually made in a radial plane by means of a peculiar bend or twist at the back end as shown in Fig
Sometimes 4 or 6 or 8 coil sides are used in each slot in several layers because it is not practicable to have too many slots (Fig ).

38. Degree of Re – Entrant of an Armature Winding
A winding is said to be single re-entrant if on tracing through it once, all armature conductors are included on returning to the starting point.
It is double re-entrant if only half the conductors are included in tracing through the winding once and so on.

39. Multiplex Winding
In such windings, there are several sets of completely closed and independent windings. If there is only one set of closed winding, it is called simplex wave winding. If there are two such windings on the same armature, it is called duplex winding and so on. The multiplicity affects a number of parallel paths in the armature.

40. Lap and Wave Windings
Two types of windings mostly employed for drum-type armatures are known as Lap Winding and Wave Winding.
The difference between the two is merely due to the different arrangement of the end connections at the front or commutator end of armature.
Each winding can be arranged progressively or retrogressively and connected in simplex, duplex and triplex.

41. Simplex Lap – Winding
It is shown in Fig which employs single-turn coils.
In lap winding, the finishing end of one coil is connected to a commutator segment and to the starting end of the adjacent coil situated under the same pole and so on, till and the coils have been connected.
This type of winding derives its name from the fact it doubles or laps back with its succeeding coils.
Contd….

42. Simplex Lap – Winding

43. Numbering of Coils and Commutator Segments
In the d.c. winding diagrams to follow, we will number the coils only (not individual turns).
The upper side of the coil will be shown by a firm continuous line whereas the lower side will be shown by a broken line.
The numbering of coil sides will be consecutive i.e. 1, 2, etc. and such that odd numbers are assigned to the top conductors and even numbers to the lower sides for a two-layer winding.
The commutator segments will also be numbered consecutively, the number of the segments will be the same as that of the upper side connected to it.

44. Simplex Wave Winding
It is clear that in lap winding, a conductor (or coil side) under one pole is connected at the back to a conductor which occupies an almost corresponding position under the next pole of opposite polarity (as conductors 3 and 12).
Conductor No. 12 is then connected to conductor No. 5 under the original pole but which is a little removed from the initial conductor No. 3.
If, instead of returning to the same N-pole, the conductor No. 12 were taken forward to the next N-pole, it would make no difference so far as the direction and magnitude of the e.m.f. induced in the circuit are concerned.

45. Dummy or Idle Coils
These are used with wave-winding and are resorted to when the requirements of the winding are not met by the standard armature punchings available in armature-winding shops.
These dummy coils do not influence the electrical characteristics of the winding because they are not connected to the commutator.
They are exactly similar to the other coils except that their ends are cut short and taped.
Contd….

46. Uses of Lap and Wave Windings
The advantage of the wave winding is that, for a given number of poles and armature conductors, it gives more e.m.f. than the lap winding. Conversely, for the same e.m.f., lap winding would require large number of conductors which will result in higher winding cost and less efficient utilization of space in the armature slots.
Another advantage is that in wave winding, equalizing connections are not necessary whereas in a lap winding they definitely are. It is so because each of the two paths contains conductors lying under all the poles whereas in lap-wound armatures, each of the P parallel paths contains conductors which lie under one pair of poles.

47. Types of Generators
Generators are usually classified according to the way in which their fields are excited. Generators may be divided into
Separately-excited generators are those whose field magnets are energised from an independent external source of d.c. current. It is shown diagramatically in Fig
Self-excited generators are those whose field magnets are energised by the current produced by the generators themselves. Due to residual magnetism, there is always present some flux in the poles.

48. Brush Contact Drop
It is the voltage drop over the brush contact resistance when current passes from commutator segments to brushes and finally to the external load. Its value depends on the amount of current and the value of contact resistance. This drop is usually small and includes brushes of both polarities. However, in practice, the brush contact drop is assumed to have following constant values for all loads.
0.5 V for metal-graphite brushes.
2.0 V for carbon brushes.

49. Generated E.M.F. or E.M.F. Equation of a Generator
Let Φ = flux/pole in weber
Z = total number of armature conductors
= No. of slots x No. of conductors/slot
P = No. of generator poles
A = No. of parallel paths in armature
N = armature rotation in revolutions per minute (r.p.m.)
E = e.m.f. induced in any parallel path in armature
Generated e.m.f. Eg = e.m.f. generated in any one of the parallel paths i.e. E.

50. Iron Loss in Armature
Due to the rotation of the iron core of the armature in the magnetic flux of the field poles, there are some losses taking place continuously in the core and are known as Iron Losses or Core Losses. Iron losses consist of
Hysteresis loss
Eddy Current loss

51. Total Loss in a D.C. Generator
The various losses occurring in a generator can be sub-divided as follows :
Copper Losses
Magnetic Losses
Mechanical Losses

52. Stray Losses
Usually, magnetic and mechanical losses are collectively known as Stray Losses.
These are also known as rotational losses for obvious reasons.

53. Constant or Standing Losses
As said above, field Cu loss is constant for shunt and compound generators. Hence, stray losses and shunt Cu loss are constant in their case. These losses are together known as standing or constant losses WC.
Hence, for shunt and compound generators,
Armature Cu loss Ia2Ra is known as variable loss because it varies with the load current.
Total loss = variable loss + constant losses WC

54. Power Stages
Various power stages in the case of a d.c. generator are shown below :
Following are the three generator efficiencies :
Mechanical Efficiency
Electrical Efficiency
Overall or Commercial Efficiency

55. Condition for Maximum Efficiency
Generator output = VI
Generator input = output + losses